1. Chapter 7
LEAVES
I. Introduction
A. Features
1. Leaves are "solar collectors"
2. Disposable (every autumn) and renewable (every spring)
B. Functions
1. Photosynthesis: fixing carbon dioxide and water into carbohydrates using sunlight and leaf pigments
2. Transpiration: most water absorbed by the roots are conducted by the xylem evaporates when it reaches the leaf.

2. II. Leaf Arrangements and Types
A. Variations
1. Sizes: duckweeds (leaves 1 mm wide) to palms (6 meters long)
2. Shapes: flattened, tubular, feathery, cup-shaped,
spine-like, needle-like
3. Textures: smooth, hairy, slippery, sticky, waxy, glossy
B. Originate as Primordia: Consist of:
1. Petiole (stalk): petiole may be absent, called sessile leaf
2. Blade (lamina): network of veins (vascular bundles)
3. Stipules (optional): appendages found at the base of a leaf
C. Arrangement on Stem (Phyllotaxy)
1. Alternate (spiral): one leaf per node
2. Opposite: two leaves per node
3. Whorled: three or more leaves per node

3. D. Simple and Compound Leaves
1. Simple leaf: undivided blade
2. Compound leaves (blade divided into leaflets)
a. Pinnately compound: leaflets in pairs along a central rachis
b. Palmately compound: leaflets attached at top of petiole
E. Arrangement of Veins (Venation)
1. Pinnately veined: single main vein called the midrib and secondary veins branch out from it
2. Palmately veined: several veins fan out from base of blade. In monocots (grasses), palmate veins are parallel In dicots, palmate veins diverge from one another (maple)
3. Dichotomous venation: veins fork evenly and progressively from base of blade (Ginkgo leaf)

5. III. Internal Structure of Leaves
A. Epidermis
1. Single, colorless layer of cells covering entire leaf
2. Cuticle present on epidermis: waxy substance called cutin
3. Stomates (or stomata) present in epidermis
4. Bulliform (motor) cells: cause leaf to fold or roll inward
5. Glands
a. Often secrete sticky substances
b. Occur as depressions, protuberances, and hairs or tips of hairs
B. Mesophyll
C. Veins: system of vascular bundles

7. Ligustrum cross section showing bulliform cells

8. IV. Stomata
A. Features
1. Tiny pores in epidermis
2. Occur on both upper and lower epidermis
3. Distribution of stomata variable depending on plant
a. Dicots: more in lower epidermis
b. Monocots: equal distribution on both surfaces
c. Aquatic plants (e.g., water lily), on upper epidermis only
B. Structure and Function
1. Guard cells
• 2 kidney bean shaped cells
2. Permit gas exchange between inside and outside of leaf and also allow water loss from inside the plant by transpiration. Guard cells carry chloroplasts and can photosynthesize. Guard cells expand when their water content increases and this causes the stomata to open. When their water content decreases they contract and close the stomata opening.

10. V. Mesophyll and Veins
A. Types of Mesophyll
1. Palisade: are upper mesophyll, tightly packed, chloroplast filled cells and function mainly in photosynthesis
2. Spongy: are upper mesophyll, loosely arranged parenchyma; air spaces between cells. Surface of cells wet with moist air.
B. Veins (Vascular Bundles)
1. Xylem and phloem (xylem oriented toward upper epidermis) surrounded by a layer of bundle sheath.
2. Bundle sheath: layer of parenchyma and/or fibers which surround veins
3. Arrangement
a. Netted veins (Dicots)
b. Parallel veins (Monocots)

11. Oleander leaf cross section showing vascular bundles

12. VI. Specialized Leaves
A. Shade Leaves
• thinner, have fewer hairs than "sun" leaves of same tree, fewer mesophyll layers and fewer chloroplasts
B. Leaves of Arid Regions (Xerophytes)
1. Thick, leathery leaves (hypodermis present)
2. Few stomata (or sunken stomata)
3. Dense hairy coverings
4. Thick cuticle
C. Tendrils
In compound leaves, the upper leaflets form tendrils that are very sensitive to touch. If touched, cells on the opposite side divide and grow very fast causing the tendril to curl in the direction of contact
Tendrils function in support

13. Acer (maple) sun (above) and shade (below) leaves

14. Vicia (Fabaceae) leaf tendrils

15. D. Spines, Thorns, and Prickles
1. Spines (modified leaves)
a. Reduction in leaf size reduces water loss from the plant
b. Protection from herbivores
2. Thorns (modified stems): arise in the axils of leaves
3. Prickles (neither leaf nor stem)
a. Outgrowths from epidermis or cortex as in roses and raspberries.
E. Storage Leaves
1. Succulent leaves, retain water in large, thin-walled parenchyma with no chloroplasts.
2. Fleshy leaves (onion, lily bulbs); store carbohydrates

16. Gleditsia triacanthos (Fabaceae: honey locust) branch with stem thorns.

17. Utrica dioica (nettle) with stinging hairs

18. F. "Flower Pot" Leaves
1. Urn-like pouches that become home to ant colonies
2. Ants carry in soil and add waste products
3. Urn-like "flower pot" then produces adventitious roots
G. Window Leaves
1. Plants of Kalahari Desert (South Africa)
2. Leaves shaped like ice cream cones which are buried in the sand
3. Dime-sized top of the "cone" is exposed at the surface
4. Transparent epidermis (membrane) allows light to enter the "window"
5. Adaptation to the drying winds of the desert

19. Frithia (Crassulaceae) with window leaves.

20. H. Reproductive Leaves
• produce plantlets from tips
I. Floral Leaves (Bracts)
1. Found at base of flowers or flower stalk
2. Poinsettia, dogwood, examples
J. Insect-trapping Leaves
1. Pitcher plants
2. Sundews
3. Venus flytraps
4. Bladderworts

21. Kalanchoe (air plant) leaf with plantlets being produced along the margins.

22. Bouganvillea (Nyctaginaceae) flowers surrounded by showy bracts

23. Drosera (sundew) insectivorous leaves

24. Dionaea (Dionaeaceae: Venus’ flytrap) leaves modified to capture insects

25. VII. Autumnal Changes in Leaf Color
A. Pigments Responsible for Coloration
1. Plastid pigments
a. Chlorophylls (green)
b. Carotenoids
1) Carotenes (yellow)
2) Xanthophylls (pale yellow)
2. Vacuolar Pigments (water soluble)
a. Anthocyanins (reds and blues)
b. Betacyanins (reds)

26. B. Mechanism of Fall Color Development
A reduction in daylight causes the green chlorophyll to break and the yellow color of carotenoids and pale yellow color of xanthophylls to predominate.
Water soluble anthocyanins accumulate in the vacuoles. If the cell sap is acidic it turns red, if its alkaline it turns blue.

27. VIII. Leaf Abscission
At the abscission zone near the base of the petiole two layers of cells are differentiated due to hormonal changes associated with aging.
One layer (several cells thich) at the stem side forms the protective layer whose cells are coated with suberin.
On the leaf side, a separation layer is differentiated. The cells swell and become gelatinous. Due to seasonal changes (low temperature, shorter days, less light intensity) the pectins of the middle lamella break down by enzymes making the attchement of leaf to stem very loose. Wind or rain can then break the leaf off the stem.

28. Prunus (Rosaceae) leaf abscission zone (longitudenal section in petiole).

29. Leaf Abscission Zone